This invention pertains generally to airborne radar systems and particularly to radar systems of such type which are adapted to perform more than one function.
It is known in the art that so-called multi-mode radar systems (meaning systems that may perform different functions, either simultaneously or in a rapid sequence) incorporate directional antennas which may be required to scan in many different ways. If such a system is to be airborne, as by a high performance aircraft, the problem of providing a satisfactory scanning technique is particularly difficult to solve. In such an application, the location of a directional antenna is, for aerodynamic reasons, restricted to the interior of a streamlined radome making up the nose section of the aircraft. With a scanning antenna so located, the limit of the scanning field of a mechanically scanned beam is in the order of 60.degree. from the longitudinal centerline of the aircraft. A scanning field of such limited size is too small for many modes of operation. Further, if rapid scanning in azimuth and elevation is required, it is necessary to provide a relatively large, heavy and powerful mechanical scanning mechanism. Such a scanning mechanism, obviously, is detrimental to the optimum capability of the radar and the aircraft.
If a mechanical scanning mechanism is replaced by any known electronic scanner (to permit rapid scanning), other types of problems are encountered. For example, because the width of the beam from a phased array antenna increases with scan angle, antenna gain decreases. Thus, at a scan angle of say 60.degree., the beamwidth doubles as compared to the beamwidth at broadside. Nevertheless, because a beam from a phased array antenna may be scanned so much more quickly than the beam from a mechanically scanned directional antenna, some kind of phased array antenna is required for multi-mode airborne radar.
If a phased array antenna is to be mounted in a streamlined radome in a high performance aircraft, several problems unique to such an installation are encountered. First, it is necessary, to avoid the occurrence of grating lobes within the scanning field, to place the individual antenna elements of a phased array as closely together as possible. Further, the type of feed used to illuminate a phased array is important, it being necessary to use some kind of constrained back feed in order to avoid antenna blockage. Any known "space fed" system must be folded to fit inside the radome, thereby creating subsequent alignment and efficiency problems; and any known "radial feed" prevents optimum disposition of the antenna elements in the array.
The difficulties mentioned hereinbefore are multiplied when operational requirements dictate that the radar in an aircraft combine high power and angular discrimination capabilities. To meet power requirements, a maximum amount of radio frequency energy, (concomitant with a satisfactory beam shape) must be radiated from each one of the antenna elements. To permit such a maximum amount of radio frequency energy to be radiated, it is necessary, in the present state of the art, to cool the antenna elements and associated control circuitry. Such cooling must be as effective at high as at low altitudes, with the result that a positive way of cooling at any operational altitude be provided. To meet both requirements, the radar beam must be narrow and well formed, implying that there be a large number of antenna elements and that the power to each be controllable. To meet angular discrimination requirements for many applications it is highly desirable that the radar be a monopulse radar. Any known constrained feed for a monopulse radar entails the extensive use of waveguide transmission lines and conventional couplers. The resulting feed is intolerably heavy and critical to adjust. Such deficiencies, when the array is to be of any appreciable size, make it infeasible to use a conventional corporate feed.